I don't think there's a set rule, any player is available for the right deal, but some players would take a lot more assets to entice Klentak to pull the trigger.
I think one "soft" rule is when you've built up minor league depth to the point where the 40 man rule constrains you, you no longer look to add prospects, but focus on adding "blue chip" prospects.
This changes the dynamic of trading, when you're building depth, Giles type trades make sense, when you have depth, trading young ML players for prospects no longer makes sense - it should take an overpay to pry Neris away because he has current and future value, and adding depth to the system now has less value (since you'll lose some lesser prospects, it's similar to including them in the deal).
So when you look at trading ML assets, you want either similar quality of assets at a position of need or blue chip prospects with an upside significantly higher than the player being traded (because there's also a real probability of failure). So you don't trade Hernandez or Rupp for a prospect with a 2 WAR ceiling, you want a prospect with a 4-5 WAR ceiling. Nor do you trade a year early and leave a gapping hole and hope a prospect might fill it, this isn't the NBA, the value of losing 5 more games in baseball is pretty small, but the impact on the team and fans (and revenue) can be significant if it creates the impression of running in place. Unless a starter is on the verge of free agent or 30+ years old, there's no rush to trade them, a proven track record raises their value, and if they're only seen as a 1-2 WAR player, then their trade value isn't that high and holding them has a low opportunity cost.
To me the most obvious trade is young pitching for young corner OFs, but that might wait a year both to see how the young pitchers develop (arm injuries, etc.) and know who's both surplus and still has value, and how the young OFs develop (is this really a need for 2018? 2020?).
Of course, if someone makes "an offer you can't refuse" well, pull the trigger!
